1. Field of the Invention
The present invention relates to an optical recording medium wherein information signals recorded on the disk surface thereof are optically read out, and specifically relates to an optical recording medium such as a video disk, a compact disk and so forth.
2. Description of the Prior Art
A conventionally adopted optical recording medium such as an optical video disk, a compact disk or the like has been constructed such that an information signal recorded surface which is provided on a transparent synthetic resin plate and formed with a plurality of information signal pits is covered by a light reflective film, wherein the information signals are reproduced by scanning the recorded surface of the synthetic resin plate by a photo detector so as to detect the existence and size of the information signal pits in accordance with a variation of the magnitude of the lights reflected therefrom.
It has been one of the most important factors in the conventional type optical recording medium constructed as above to form a light reflective film with a material of high reflectivity, and in many cases aluminum having high-degree of purity has been applied by a vapor deposition. However, the reflective film made of this aluminum with high-degree of purity is not substantially stable, and thus it has been likely to be deteriorated even by oxygen, water moisture and so on in the air, and the film surface is peeled off to thereby cause a blister, pin holes and so forth, resulting that erroneous fault in reading information signals is increased. For this reason, a protection coating layer or the like has been applied to the rear surface side of the light reflective film to prevent the reflective film from deteriorating by oxidation, but it was not substantially effective to perfectly protect it from the deterioration due to an exfoliation or the like.
There have been proposed various methods to improve the peeling phenomenon of the light reflective film so far, a case in point being the one disclosed in Japanese Patent Application Laid Open No. 57-186244, wherein it is proposed to form a film layer through an evaporation or a spattering of a metal alloy of specific components.
In this method, it is disclosed that a light reflective film having a high durability can be obtained by an evaporation or a spattering of a metal alloy which is selected from the group consisting of an Al--Cr alloy having more than 75 at.% aluminum, an Al--Cu alloy having more than 40 at.% aluminum, an Al--B alloy having more than 70 at.% aluminum, an Al--Mn alloy having at least 75 at.% aluminum, and an Al--lanthanide alloy having 0.1 to 3.0 at.% of the lanthanide, wherein each of these metal alloys comprises at least 1 at.% of the composing elements, with the exception of the Al--lanthanide alloy. However, an optimum durability cannot be obtained if an enough quantity of the second composing element is not contained in the metal alloys, and on the other hand, if a quantity of the second composing element therein becomes too much, the reflectivity thereof is also lowered, so that it has been a difficult problem to obtain a reflective film with a high durability, yet with a high reflectivity altogether.
It is also widely known that a reflective film which is not easily offended by a peeling phenomenon or the like can be obtained if the evaporation of an aluminum or an aluminum metal alloy onto the signal recorded surface is performed at a speed not higher than 1.5 nanometers per second as disclosed in Japanese Patent Publication No. 3-79775. However, since the film layer forming speed is so low in this method, it is difficult to raise the productivity thereof, and to control a stability of the evaporating speed, and also there is a problem that even when a reflective film with high reflectivity is obtained, the durability thereof is not stable due to a possible variation of the evaporating speed.